1. Field of the Invention
The present invention relates to a metal halide lamp suitable as a light source for a vehicle headlight and/or infrared night imaging vision apparatus, and a metal halide lamp lighting apparatus using the metal halide lamp.
2. Description of the Related Art
Various researches have been made concerning the safety of vehicles. See, for example, “Illuminating Engineering Institute Journal”, Vol. 86, No. 12, pp. 896-899, published 2002. This document discloses an infrared night imaging vision apparatus for vehicles as vehicle safety means. Infrared night imaging vision apparatuses for vehicles are called “Night Vision” (trademark), and developed as nighttime safety drive support systems for drivers utilizing the properties of infrared rays, to enhance the visibility of pedestrians, obstacles or traffic signs ahead of a vehicle. In 1999 in the US, an infrared night imaging vision apparatus was introduced to the market for the first time. An obstacle, for example, that is a long way away and cannot be detected using headlights is photographed using an infrared camera, and its image is displayed for a driver. Infrared light has longer wavelengths than visible light. Therefore, when detecting an obstacle, for example, at night in the rain or mist, it is more advantageous for a driver to acquire an image of the obstacle using infrared light, than to directly see it using visible light. Further, the driver can detect an obstacle from its image acquired using infrared light, even if, for example, they are dazzled by light emitted from the headlights of an oncoming vehicle.
Infrared night imaging vision apparatuses for vehicles include passive ones and active ones. Passive apparatuses detect, using a far-infrared camera, far-infrared light (with wavelengths of 8-14 μm) emitted from an obstacle. Apparatuses of this type are disadvantageous in that the camera is expensive and its accuracy of detection is degraded when it rains or snows. In contrast, active apparatuses emit near-infrared light to an obstacle using a projector, and detect reflected light using a CCD camera that senses near-infrared light. Further, a conventional light source for infrared night vision projectors is formed of a combination of a halogen bulb and wavelength correcting filter, and projects near-infrared light of 780 nm to 1.2 μm. Apparatuses of this type are advantageous in that the camera is not expensive and provides images near visible light ones. In apparatuses of both types, the detected images are displayed on a head-up or head-down display.
In active apparatuses, a lamp unit is known which is provided with a discharge tube containing a halide of cesium, and a near-infrared transmission filter on the tube, the discharge tube and filter being used as a light source for the infrared night imaging vision apparatus. See, for example, Jpn. Pat. Appln. KOKAI Publication No. 2003-257367. The lamp unit disclosed in this document emits near-infrared light by discharge, using either cesium iodide or cesium bromide. This near-infrared light is extracted by the near-infrared transmitting filter surrounding the lamp. Thus, the near-infrared light is intended to be dedicated to the infrared night imaging vision apparatus. Further, the document also discloses a technique for enabling the near-infrared transmitting filter to be retracted from around the discharge tube, thereby making the lamp also usable as a vehicle fog lamp. That is, the document describes that the lamp unit can also be used as a fog lamp when it is used as a light source dedicated to the night imaging vision apparatus. This lamp unit, however, cannot be used as a vehicle headlight.
As described above, vehicle infrared night imaging vision apparatuses of the active type are advantageous compared to passive ones. However, apparatuses of the active type need to use a dedicated light source at least when they are used as night imaging vision apparatuses. This being so, it is necessary to prepare a light source dedicated to the infrared night imaging vision apparatus, in addition to a vehicle headlight, or to prepare a complex fog lamp with a movable section. As a result, they become expensive.
In contrast, the inventor of the present invention has previously developed, as an embodiment of an invention, a metal vapor discharge lamp including a light source for both a vehicle headlight and infrared night imaging vision apparatus. This invention was filed as Jpn. Pat. Appln. No. 2002-294617 (hereinafter referred to as “the prior invention 1” for facilitating the explanation). Further, the inventor has proposed a 35-watt mercury-free metal halide lamp for both a vehicle headlight and infrared night imaging vision apparatus in Jpn. Pat. Appln. No. 2003-377813 (hereinafter referred to as “the prior invention 2” for facilitating the explanation).
In the lamp unit and lamps described in the above-mentioned patent document and prior inventions 1 and 2, alkali metals such as sodium (Na), potassium (K), rubidium (Rb) and cesium (Cs) are mainly used for the emission of near-infrared light. These alkali metals, which are sealed as metal halides, emit lines of the following wavelengths at the near-infrared region:
Na: 818.3 nm, 819.4 nm, 1138.1 nm, 1140.1 nm
K: 766.4 nm, 769.8 nm, 1168.9 nm, 1177.1 nm
Rb: 761.9 nm, 775.7 nm, 775.9 nm, 780.0 nm, 794.7 nm, 887.3 nm
Cs: 760.9 nm, 801.5 nm, 807.9 nm, 852.1 nm, 876.1 nm, 894.3 nm, 917.2 nm, 920.8 nm, 1002.0 nm, 1012.0 nm.
Although in the patent document and prior inventions 1 and 2, the above alkali metals are sealed as metal halides, they exist in the form of neutral metals or ions during lighting of the lamps. Alkali metals have only one electron in the outermost orbit, therefore can be very easily ionized. Accordingly, they are liable to move through the material of a hermetic vessel when a voltage is applied. This tendency is especially strong in Li or Na which have a small atomic radius. The phenomenon of movement of Li or Na atoms in the material of the hermetic vessel is known as a Li or Na dropout. The same tendency is also seen in K, Rb and Cs. Because of this, a reduction in the quantity of such a metal in the hermetic vessel is observed during long-term lighting.
This phenomenon raises a problem in which the energy of emission of near-infrared light is reduced during long-term lighting of a metal halide lamp. Therefore, when the near-infrared light of a metal halide lamp is mainly utilized, the life span of the lamp as a near-infrared source is shortened. However, a more serious problem is raised if the visible light and near-infrared light of a metal halide lamp are simultaneously utilized. In this case, the emission power maintenance ratio of near-infrared light is significantly reduced compared to that of visible light. As a result, the monitoring range of the infrared night imaging vision apparatus is decreased because of the reduction of the near-infrared emission power maintenance ratio, although the lamp has a long life as a light source for a headlight. This shortens the actual life of the lamp.
The above problems become more serious if as in prior invention 2, the initial luminous flux must be kept within a predetermined range. If the energy of near-infrared light that occupies the entire quantity of emission is increased, that of visible light is relatively reduced. Accordingly, to keep the total luminous flux within a predetermined range, the emission power of near-infrared light cannot be set high.
It is known that a so-called HID headlight that uses a metal halide lamp as a visible light source is a very bright lamp. Therefore, a good deal of reduction in total luminous flux is permitted. According to Japan Electric Lamp Manufacturers Association Regulation JEL215 1998, it is sufficient if 60% or more of the original total luminous flux is maintained after the lamp has been lit for 1500 hours. In contrast, in the case of a metal halide lamp for infrared night imaging vision apparatuses, the emission power of near-infrared light is kept low at and after the initial stage of lighting as described above. Therefore, if a significant reduction in near-infrared light output occurs during long-term lighting, the visibility performance of the infrared night imaging vision apparatus itself may well disappear.